Summary This is an external filament holder that fits the small 120mm spools. The double bearings allow the narrow spools to ride in the gap between the two bearings. This helps with keeping the spool centered. You can print bolts and nuts here if you want. http://www.thingiverse.com/thing:193647 Instructions I printed this at 100% fill. Parts print one holder 8 x 608 bearings. http://www.amazon.com/gp/product/B002BBGTK6?psc=1&redirect=true&ref_=oh_aui_detailpage_o00_s00 12 x M8 flat washers 2 x M8 bolts. The bolts need to be 65mm long 6 x M8 nuts You can remove the washers between the bearings if you want. It helps reduce friction on the spools without the washers. Installation slide bolt into base then washer onto bolt then bearing onto bolt then washer onto bolt then bearing onto bolt then washer onto bolt then tighten a nut onto the bolt then tighten another nut onto the bolt then washer onto bolt then bearing onto bolt then washer onto bolt then bearing onto bolt then washer onto bolt then slide bolt into other side of base then attach nut to bolt I put washers on the outside of the base too, but you don't need to do that.